1. Field of the Invention
This invention relates to an optical amplifier evaluation method and an optical amplifier evaluation instrument for evaluating the gain and noise figure of an optical amplifier when the optical amplifier amplifies a wavelength multiplexed signal light beam (or frequency multiplex signal light beam) provided by multiplexing a plurality of signal light beams different in wavelength (namely, frequency) in an optical signal.
2. Description of the Related Art
First, the measuring principle and a measuring method of the gain and noise figure of an optical amplifier in an optical amplifier evaluation instrument having been used hitherto.
Gain [Gn] and noise figure [NFn] when the optical amplifier amplifies a multiplex signal (WDM: Wavelength Division Multiplex) of signal light beam having one wavelength (namely, frequency) or signal light beams output by n light sources (wavelengths xcex1 to xcexm) different in wavelength (frequency) having been introduced heavily commercially in recent years are found according to expressions (3) and (4) respectively.                               G          n                =                                            P                              out                ⁢                                  xe2x80x83                                ⁢                _                ⁢                                  xe2x80x83                                ⁢                n                                      -                          P                              ASE                ⁢                                  xe2x80x83                                ⁢                _                ⁢                                  xe2x80x83                                ⁢                n                                                          P                          in_              ⁢                              xe2x80x83                            ⁢              n                                                          (        3        )                                          NF          n                =                                            P                              ASE_                ⁢                                  xe2x80x83                                ⁢                n                                                    h              ⁢                              ·                n                            ⁢                              G                n                            ·                              ·                n                                              +                      1                          G              n                                                          (        4        )            
where n is 1 to m.
[Pinxe2x80x94n] is light power of signal light beam input to the optical amplifier, [Poutxe2x80x94n] is output light power of amplified signal light beam output from the optical amplifier, [PASExe2x80x94n] is amplified spontaneous emission power output from the optical amplifier in the wavelength that the signal light beam has, [xcex94xcexdn] is an optical signal light passage band width of a light intensity measuring instrument for measuring the amplified spontaneous emission power [PASExe2x80x94n], [xcexdn] is the frequency proper to the signal light beam input to the optical amplifier, and [h] is a Planck""s constant.
The suffix xe2x80x9cASExe2x80x9d is an abbreviation for Amplified Spontaneous Emission and refers to amplification based on a so-called spontaneous emission process in which excited atoms spontaneously emit light independently of the external effect and make a transition to any other stationary energy state.
However, to find the noise figure [NFn] using the above-mentioned expression (4), it is difficult to directly find the noise figure [NFn] because generally the output light power of amplified signal light beam [Poutxe2x80x94n] is superposed on the amplified spontaneous emission power [PASExe2x80x94n] for output. Then, in a related art, the noise figure [NFn] is measured according to the following method:
The measuring method of the noise figure [NFn] in the related art will be discussed in detail.
FIG. 15 is a block diagram to show the configuration of an optical amplifier evaluation instrument and an optical amplifier evaluation method in the related art. That is, it is a diagram to describe the method of measuring the gain and noise figure of an optical amplifier in the related art. In the figure, signal light beams from light sources 101a, 101b, 101c, . . . , and 101n different in wavelength (frequency) are combined by an optical combiner 102, then the resultant signal light beam is pulse-intensity-modulated by a first optical modulator 103 and is input through an input optical terminal 108 to a measured optical amplifier 107. The input signal light spectrum at this time is a wavelength multiplexed input signal light spectrum 110 shown in FIG. 16, and the first optical modulator 103 is controlled by a pulse signal [a] output from a modulation signal generation section 105.
The input signal light beam is amplified and output from the measured optical amplifier 107. Since the input signal light beam is pulse-modulated, the amplified signal light beam output undergoes a propagation delay of the measured optical amplifier 107 and is shifted in phase, but is produced in a pulse state in the same period. The above-mentioned amplified spontaneous emission is output regardless of the presence or absence of pulse. At this time, the pulse modulation period is a period sufficiently shorter than the atomic lifetime at upper level of the amplification medium of the measured optical amplifier 107 or the carrier lifetime, so that the amplified spontaneous emission (generally called xe2x80x9cASExe2x80x9d) becomes an almost constant light output level regardless of whether the input signal light beam is on or off. The output light spectrum of the measured optical amplifier 107 becomes a waveform like an output light spectrum 111 shown in FIG. 16. FIG. 16 is a drawing to show the wavelength multiplexed signal light beam amplification form of the measured optical amplifier in the related art.
Output light of the measured optical amplifier 107 is input through an output optical terminal 109 to a second optical modulator 104. The second light modulator 104 is controlled by a pulse signal [b] output from the modulation signal generation section 105. The first optical modulator 103 and the second optical modulator 104 are driven in the same period and the phase of the second optical modulator 104 can be arbitrarily set in the 360-degree range based on the modulation timing of the first optical modulator 103.
First, the input optical terminal 108 and the output optical terminal 109 are previously connected directly as 108xe2x80x2 and 109xe2x80x2 and the light power [Pinxe2x80x94n] for each frequency [xcexdn] of signal light beam input to the measured optical amplifier 107 is measured by a light intensity measuring instrument 106. The spectrum at this time is exactly as 112 in FIG. 17. FIG. 17 is a drawing to show each light power measured in the related art as spectrum display.
Next, the input optical terminal 108 and the output optical terminal 109 are connected to the measured optical amplifier 107 and the light power [Poutxe2x80x94n] of the amplified signal light beam output from the measured optical amplifier 107 is measured for each frequency. The spectrum at this time is exactly as 113 in FIG. 17. The phases of the first optical modulator 103 and the second optical modulator 104 are the relationship between A (modulation timing of first optical modulator) and C (timing of second optical modulator when post-amplified signal light beam [Poutxe2x80x94n] is measured), and the propagation time of a waveguide of the measured optical amplifier 107 appears as a delay. FIG. 18 is a drawing to show the relative phase relationships among the first modulator, pulse light output by the measured optical amplifier, and the second modulator in the related art.
Next, the phase of the second modulator 104 is shifted 180 degrees with respect to C (timing of second optical modulator when post-amplified signal light beam [Poutxe2x80x94n] is measured) like the relationship between A (modulation timing of first optical modulator) and D (timing of second optical modulator when amplified spontaneous emission power [PASExe2x80x94n] is measured), and amplified spontaneous emission power [PASExe2x80x94n] output by the measured optical amplifier 107 is measured for each frequency. The amplified spontaneous emission spectrum at this time is exactly as waveform 114 in FIG. 17.
The measurement values are assigned to the above-mentioned expressions (3) and (4), whereby the gain and noise figure of the measured optical amplifier 107 can be calculated and found.
FIG. 19 shows the characteristics of the gain and noise figure of the measured optical amplifier 107 at the wavelength multiplexed signal light amplification time, found by the above-mentioned calculation, namely, shows wavelength characteristic of the gain, 115, and wavelength characteristic of the noise figure, 116.
As described above, in the related art, measurement is executed as many as the number of wavelengths of the light sources multiplexed and thus the measurement result is plotted discretely on the wavelength axis. Therefore, to increase the number of plots and provide continuous data, it is necessary to provide as many light sources as required for the purpose and multiplex. Such a measuring technique is disclosed in JP-A-09-018391 already proposed by the inventor et. al.
The optical amplifier evaluation method according to the related art is very effective for evaluating the gain and noise figure for each channel in a previously fixed known single-wavelength optical signal or multiple-wavelength multiplexed optical signal. However, to evaluate an optical amplifier in multiple-wavelength multiplexed optical signal, as many light sources as the required number of wavelengths need to be provided, leading to a large-scaled and expensive evaluation instrument. The wavelength in each channel is fixed and the band for each channel of an optical combiner is also limited and it is hard to vary the wavelength in a wide range, thus it is difficult to evaluate the gain and noise figure characteristics in any desired wavelength between channels, for example, other than the wavelengths proper to the light sources provided.
To solve the problem of providing a large number of light sources, a probe method of decreasing the number of channels of the light sources input to a measured optical amplifier as compared with that under the actual operating condition and applying the homogeneous characteristic of an optical amplifier is proposed. In the probe method, measurement is executed according to the following method: The measured optical amplifier is saturated by inputting a few number of channels (one to several channels) and the same value of the total signal light power as the total light power of actually used multiplex signal light. Further, probe signal of tunable laser source or EELED, etc., is input with low input light power to such an extent that the measured optical amplifier is not affected. To input the source as the probe signal light, the probe signal light wavelength is set in accordance with each wavelength of actual WDM signal light, and output signal light power and output amplified spontaneous emission power of the measured optical amplifier in the state are measured, whereby the gain and noise figure characteristics in the actual use state are calculated.
However, in the probe method, the gain characteristic of the measured optical amplifier changes depending on the setup condition of the wavelength (namely, frequency) of signal light input to saturate the measured optical amplifier and thus it is difficult to set the optimum condition for maintaining the homogeneous characteristic. If the optical frequencies of probe signal light and signal light input to the measured optical amplifier to saturate the measured optical amplifier are brought close to each other, the gain characteristic and the noise figure characteristic change because of a spectral hole burning phenomenon; this is a problem. It is hard for the method to precisely evaluate the measured optical amplifier.
It is therefore an object of the invention to provide an optical amplifier evaluation method and an optical amplifier evaluation instrument capable of measuring the characteristic of a measured optical amplifier easily and with high accuracy without a measurement error caused by the measuring person and capable of continuously measuring noise figures of a large number of measured optical amplifiers.
To the end, according to a first aspect of the invention, pulse intensity modulation is executed for a rectangular spectrum light source for providing high-level light output having a flat characteristic at an output level in an arbitrary wavelength (namely, frequency) range of an optical signal and a rectangular spectrum shape over a wide band in a sufficiently shorter period than the atomic lifetime at an upper level of the measured optical amplifier or the carrier lifetime, the on/off state of signal light is caused to exist in a constant period and with a constant width on the time axis, and the result is input to a measured optical amplifier.
The phase of the second optical modulator is controlled, the sampling window is synchronized with a time domain in which an optical pulse signal exists, and post-amplified signal light power [Poutxe2x80x94n] for each frequency component contained in the rectangular spectrum light source is measured. The sampling window is synchronized with a time domain in which an optical pulse signal does not exis, and amplified spontaneous emission power [PASExe2x80x94n] for each frequency component contained in the rectangular spectrum light source is measured. The noise figure of the measured optical amplifier, [NFn], is computed with respect to each of the wavelengths (namely, optical frequencies) according to                               NF          n                =                                            P                              ASE                ⁢                                  xe2x80x83                                ⁢                _                ⁢                                  xe2x80x83                                ⁢                n                                                    h              ⁢                              ·                n                            ⁢                              G                n                            ·                              ·                n                                              +                      1                          G              n                                                          (        5        )            
wherein a blank constant is [h], each frequency component contained in the rectangular spectrum light source undergoing the pulse modulation is [xcexdn], the gain of the measured optical amplifier at each frequency is [Gn], the optical signal light passage band width of a light intensity measuring instrument when the amplified spontaneous emission power [PASExe2x80x94n] synchronized with the time domain in which no optical pulse signal exist is measured is [xcex94xcexdn], and the sampling wavelength in a level flat portion of the rectangular spectrum light source is n=1 to m.
According to a second aspect of the invention, in the first aspect of the invention, the gain [Gn] at each wavelength (namely, frequency) of input signal light of the measured optical amplifier is calculated according to                               G          n                =                                            P                              out                ⁢                                  xe2x80x83                                ⁢                _                ⁢                                  xe2x80x83                                ⁢                n                                      -                          P                              ASE                ⁢                                  xe2x80x83                                ⁢                _                ⁢                                  xe2x80x83                                ⁢                n                                                          P                          in              ⁢                              xe2x80x83                            ⁢              _              ⁢                              xe2x80x83                            ⁢              n                                                          (        6        )            
According to a third aspect of the invention, in the first or second aspect of the invention, the fluctuations of the values dependent on the input optical frequencies of the gain [Gn] and the noise figure [NFn] of the measured optical amplifier can be found continuously with respect to a wavelength (namely, frequency) axis based on continuity of the spectrum of the wide-band rectangular spectrum light source used as the light source and slopes of the gain [Gn] and the noise figure [NFn] at any desired wavelength (namely, frequency) can be found from the result.
According to a fourth aspect of the invention, in the first or second aspect of the invention, pulse modulation light input to the measured optical amplifier is generated by pulse-modulating the rectangular spectrum light source directly by an electric pulse signal.
Further, according to a fifth aspect of the invention, in the first or second aspect of the invention, when the output light from the rectangular shape spectrum light source is a continuous wave (generally called xe2x80x9cCWxe2x80x9d) light, it is passed through the optical modulator (for example, optical switch) driven by an electric pulse signal for pulse modulation, and an optical pulse fed into input of the measured optical amplifier is provided.
According to a sixth aspect of the invention, in the first or second aspect of the invention, an optical modulator (for example, optical switch) driven by a repetitive pulse signal synchronized with an optical pulse signal supplied to the measured optical amplifier is used to separate two types of output light power of the post-amplified signal light power [Poutxe2x80x94n] and the amplified spontaneous emission power [PASExe2x80x94n] continuously output in time sequence from the measured optical amplifier.
Further, according to a seventh aspect of the invention, in the first or second aspect of the invention, two types of output light power of the post-amplified signal light power [Poutxe2x80x94n] and the amplified spontaneous emission power [PASExe2x80x94n] continuously output in time sequence from the measured optical amplifier are measured by the light intensity measuring instrument such as an optical spectrum analyzer having a gate measuring function based on timing conducts measurement.
According to an eighth aspect of the invention, in the first or second aspect of the invention, the width (that is, time) of the sampling window as the second optical modulator is turned on/off is shorter than the width or the time during which the first optical modulator is on or off and relative phase relationship such that an overlap exists in time domains preceding and following the sampling window of the second optical modulator as the first optical modulator is turned on or off is set.
Further, according to a ninth aspect of the invention, in the first or second aspect of the invention, the measured optical amplifier is a rare earth element doped optical fiber amplifier and the modulation frequency of intensity modulation light is 10 kHz or more.
According to a tenth aspect of the invention, in the first or second aspect of the invention, the measured optical amplifier is a semiconductor optical amplifier and the modulation frequency of intensity modulation light is 1 GHz or more.
Further, according to an eleventh aspect of the invention, in the first or second aspect of the invention, the optical modulator (for example, optical switch) used for pulse-modulating the signal light is an acousto-optic switch.
According to a twelfth aspect of the invention, in the first or second aspect of the invention, the optical modulator (for example, optical switch) for separating the post-amplified signal light power [Poutxe2x80x94n] and the amplified spontaneous emission power [PASExe2x80x94n] output from the measured optical amplifier is an acousto-optic modulator.
According to a thirteenth aspect of the invention, there is provided an optical amplifier evaluation instrument comprising a control and arithmetic unit for controlling the components to realize an optical amplifier evaluation method as set forth in any one of the first to twelfth aspects of the invention, performing operations on the results detected by the light intensity measuring instrument, and calculating the gain [Gn] and the noise figure [NFn] of the measured optical amplifier for making it possible to conduct automatic measurement.
According to a fourteenth aspect of the invention, the optical amplifier evaluation instrument as set forth in the thirteenth aspect of the invention further comprises an optical variable attenuator for varying pulse-modulated light power of the rectangular spectrum light source input to the measured optical amplifier and setting arbitrary input light power, wherein the optical variable attenuator is controlled by the control and arithmetic unit.
Further, according to a fifteenth aspect of the invention, the optical amplifier evaluation instrument as set fourth in the thirteenth or fourteenth aspect of the invention further comprises optical path switch means, wherein the optical path switch means is controlled by the control and arithmetic unit, whereby the light power for each frequency component input to the measured optical amplifier, [Pinxe2x80x94n], and the post-amplified signal light power [Poutxe2x80x94n] , and the amplified spontaneous emission power [PASExe2x80x94n] output from the measured optical amplifier are measured automatically.